Among the image developing apparatus for visualizing a latent image by electro-photography prcess (hereinafter referred to as electro-photography device), a dry-toner type has been forming the mainstream line. An increasing number of copying machines, laser printers, facsimile devices using ordinary paper, and the like equipment incorporate such type of electro-photography device. This is an applied device of an electro-photography process technology, in which an electrostatic latent image formed on a photosensitive medium is visualized with the use of nonmagnetic toner particles.
In view of the advantages in downsizing and cost reduction, the use of a nonmagnetic one-component developing apparatus (developing apparatus) is increasing.
An electro-photography device using a conventional developing apparatus is described below in its structure and the operation with reference to the drawings. FIG. 2 shows structure of an electro-photography device using a conventional developing apparatus.
In FIG. 2, a photosensitive medium 1 comprises a photosensitive layer coated around a metal drum. A charger 2 arranged adjacent to the photosensitive medium 1 comprises a charging wire 2a made of tungsten or the like wire, a metal shield panel 2b and a grid 2c. The charging wire 2a causes corona discharge and the ion produced by the discharge evenly charges the outer surface of the photosensitive medium 1 through the grid 2c. An exposure optical system 3 directs an exposure beam 4 onto the photosensitive medium 1 so as to form an electrostatic latent image thereon. The exposure beam 4 is subjected to light intensity modulation or pulse width modulation by means of a laser drive circuit.
A toner supply roller 6 supplies a toner 10 agitated and carried by a toner agitating member 11 on the surface of a developing roller 5, or a toner carrier. The toner supply roller 6 is connected to a toner supply bias power source 26 for obtaining a toner supply bias voltage. By delivering a voltage comprising a d.c. voltage and an a.c. voltage as the toner supply bias voltage, the transfer of toner onto the developing roller 5 and the scrape-off of unconsumed toner can be made simultaneously and efficiently. This also eliminates both of the problems, the lowering in the density at the leading end part of an image and the lowering in the density at the trailing end part of an image. The toner supply roller 6 is adapted to make contact with the developing roller 5. The developing roller 5 has its core made of metal such as stainless steel, covered with elastic material layer, and is rotatably attached to a developing hopper 9. Numeral 5a, 6a denote a shaft.
The toner 10 fed by the toner supply roller 6 is held by a toner regulating blade 7 so as to be subjected to functional electrification, and a thin layer of toner 10 is formed over the outer periphery of the developing roller 5. The developing roller 5 is adapted to make contact with or located adjacent to the photosensitive medium 1; the toner 10 is transferred onto and stuck to a part of the photosensitive medium 1 where a latent image is formed, so as to visualize the latent image. The transfer of toner 10 is conducted by the effect of a bias voltage applied by a developing bias power source 12.
The toner-regulating blade 7 is composed of a metal leaf spring member 7a and a toner regulating member 7b adapted to make contact with the outer periphery of the developing roller 5. The toner-regulating blade 7 is secured to a blade holder 8 with fastening screws. The toner 10 which has been stored in the developing hopper 9 is agitated by a toner agitating member 11 which depicts a circle in synchronization with the toner supply roller 6 in order to prevent the toner 10 stored in the developing hopper 9 from coagulating, and to feed the toner 10 onto the supply roller 6.
Sheets 15 stored in a sheet cassette 14 are taken out one by one by a semicircular roller 16, and are conveyed by a conveying roller 17. The taken-out sheet 15 is transferred by the conveying roller 17 in the direction as indicated by an arrow symbol A. A registration roller 18 once stops and holds each of the sheets 15 in order to align the sheet with a toner image formed on the photosensitive medium 1, and a driven roller 19 makes contact with the registration roller 18. A transfer roller 20 makes contact with the photosensitive medium 1. As soon as a toner image formed on the photosensitive medium 1 arrives, as a result of rotation, at a point where the transfer roller 20 and the photosensitive medium 1 are making contact to each other, the sheet 15 reaches concurrently at the point. A transfer bias power source 24 supplies a high bias voltage to a metal shaft 20a of the transfer roller 20 so as to apply a charge having a polarity reverse to that of the toner 10 onto the rear surface of the sheet 15, and accordingly, the toner image on the photosensitive medium 1 is transferred to the sheet 15.
The sheet 15 is transferred to the left in the illustration, and held between a heat roller 22 which incorporates a heat source in itself and a press roller 23 of a fixing unit 21 so that the toner image transferred on the sheet 15 is fixed. The photosensitive medium 1 after the toner image formed thereon is transferred onto the sheet 15 is cleaned by a cleaning blade 50 provided for removing residual toner, and a discharger 13 irradiates a light on the photosensitive medium 1 for removing residual charge, in preparation for a next process step.
As the image density varies due to change in the operating environments and aging in the above described electro-photography devices, an image density control means 28 is provided for automatically controlling the image density, or controlling it by hand. The image density control means 28 optimizes image density by sending a signal to a developing bias voltage controlling means 27 for changing the developing bias voltage to be applied on the developing roller 5.
When the developing bias voltage alone is changed for adjusting the image density in an electro-photography device of the above described structure, the potential between the developing roller 5 and the toner supply roller 6 also changes. Applying the superimposed voltage comprising a d.c. voltage and an a.c. voltage on the toner supply roller 6 is for supplying the toner 10 to the developing roller 5 and for scraping the unconsumed toner 10 off the surface of the developing roller 5 at a same time.
If an insufficient quantity of toner 10 is supplied to the developing roller 5, image density at the trailing end part becomes low when an image of high overall density is printed. On the other hand, an insufficient scraping of the unconsumed toner 10 results in a lowered density at the leading end part of an image. Because the toner 10 sticking on the surface of the developing roller 5 passes through under the toner regulating blade 7 for a plurality of times, as a result, the charging quantity of the toner increases. Therefore, if the developing bias is changed which bring about a change in the difference in the potential between the developing roller 5 and the toner supply roller 6, then either the insufficiency of toner 10 supply or the insufficient scraping of unconsumed toner 10 happens, and the quality of image deteriorates.
The present invention addresses the above described problems inherent to the prior art, and aims to offer a developing apparatus with which the quality of image is well preserved even if the developing bias voltage is changed for the purpose of adjusting the density of an image.